1. Field of the Invention
The present invention relates to a process for improving the adhesion properties between a first coating obtained from a curable composition and a second coating, through the incorporation of a cleavable surfactant in said curable composition and subsequent cleavage thereof. Curable compositions containing cleavable surfactants and optical articles having deposited thereon stacks of coatings formed by the above process are also described.
2. Description of Related Art
It is a common practice in the art to coat at least one main surface of a lens substrate, such as an ophthalmic lens or lens blank, with several coatings for imparting to the finished lens additional or improved optical or mechanical properties. These coatings are designated in general as functional coatings.
Thus, it is usual practice to coat at least one main surface of a lens substrate, typically made of an organic glass material, with successively, starting from the surface of the lens substrate, an impact-resistant coating (impact resistant primer), an abrasion- and/or scratch-resistant coating (hard coat), an antireflection coating and, optionally, an anti-smudge top coat. Other coatings such as a polarized coating, a photochromic coating or a dyeing coating may also be applied onto one or both surfaces of the lens substrate.
The typical formulation of a coating composition to be deposited by a wet route includes a surfactant that enhances the wettability of the surface to be coated and acts as a leveling agent. Actually, a coating composition may wet a surface unevenly and sag, which would result in thickness variations on the substrate. This is particularly true for aqueous sol-gel compositions. Including a surfactant in the composition provides a more evenly deposited coating.
However, when such surfactant-containing composition is spread and cured, the surfactant remains or migrates at the surface of the coating, resulting in a first coating which presents a low surface energy. When a second coating is applied onto said first coating, the applied coating composition does not spread correctly and/or, after curing, does not lead to adhesion between the two coatings.
This problem is traditionally solved by carrying out a preparation of the surface of the first coating before deposition of the second coating. A surface preparation is a treatment resulting in an increase of surface energy by surface physical attack and/or chemical modification. It consists in a chemical or physico-chemical attack by highly reactive species such as strong bases from alkaline solution or energetic species such as ions or radicals.
Such treatments usually activate the surface of the outermost layer of the optical article by altering the chemistry of a few outermost molecular layers. Good adhesion between two coatings generally requires strong interfacial forces via chemical compatibility and/or chemical bonding.
A surface preparation assists in creating chemically active functional groups (polar groups) at the surface of the coating, such as amine, carbonyl, hydroxyl and carboxyl groups, to improve interfacial adhesion. For instance, using an oxygen gas plasma may create hydroxyl functionality, thus increasing the wettability of the surface to be coated.
As a surface pre-treatment step, a high-frequency discharge plasma method, a glow discharge plasma method, a corona treatment, an electron beam method, an ion beam method, an acid or base treatment by using a concentrated solution and/or an immersion in such a solution can be employed.
However, a chemical or physico-chemical attack is difficult to control and to limit at the surfactant layer. It presents the risk of attacking more than the surface of the treated coating. For example, in the case of polysiloxane coatings, an alkaline attack is often used and it is well known that the chemical consequence of such treatment is a decrease of the subsurface cross-linking. In addition, surface preparation treatments raise safety problems (heated alkaline solution, corona . . . ) and increase the cost of the process (cost for surface preparation, operation and equipment) and its complexity, when a stack of several layers is needed.
Another solution to avoid obtaining a low surface energy coating is to employ a first coating composition devoid of surfactant. A careful choice of solvents is then needed to allow wettability and good spreading, because the wettability of a surface depends on the chemical nature of the surface material and the solvent choice strongly depends on the substrate and coating surface energy. In addition, this alternative solution is too restricting, since water based coating compositions can be hardly used.